A philosopher, metaphysician, and mathematician, Chauncey Wright graduated from Harvard in 1852 and taught occasionally at the College while employed as a "computer" with the American Ephemeris and Nautical Almanac. A positivist and empiricist in the British tradition, he exerted an influence on the development of American Pragmatism through his younger friends William James, Charles S. Peirce, and Oliver Wendell Holmes, but is perhaps best remembered as one of the earliest and most able defenders of Darwinism and Darwinian natural selection. The bulk of the 137 items in the Wright Papers is comprised of personal letters addressed to Wright during the adult years of his brief life. From the typical letters of a college student, the correspondence branches out to touch upon philosophy, mathematics, and Wright's meeting with Darwin in 1872. Among the more prominent correspondents are C. S. Peirce, Charles Eliot Norton, Francis Bowen, Susan and J. Peter Lesley, and James Bradley Thayer and William Sydney Thayer.

After a brief career in the Congregational church, J. Peter Lesley (1819-1903) left the ministry in 1852 to work full-time as a geologist. Having gained experience in structural geology and stratigraphy with the First Pennsylvania Geological Survey (1836-1842), Lesley became an expert in the geology of coal, working for the Pennsylvania Railroad and other corporations and conducting some of the first systematic studies of the state's hydrocarbon resources. A long-time professor at the University of Pennsylvania, he was elected to the APS in 1856, serving variously as its librarian, secretary, and vice president, and he was selected as Director of the Second Pennsylvania Geological Survey (1875-1889). The Lesley Papers include letters to and from Peter Lesley and his wife, Susan on geology, coal and iron mining, abolition, educational reform, organized charity, and Unitarianism. The collection offers important insight into academic and applied geology in late 19th century Pennsylvania, the development of the coal and iron industries, as well as into the Lesleys' progressive social and intellectual milieu. It is divided into three series: Lesley's general correspondence, his correspondence with his brother Joseph, and microfilms of Lesley's research notes.